1. Field
The disclosed concept pertains generally to electrical switching apparatus and, more particularly, to circuit breakers including electronic trip units.
2. Background Information
Circuit breakers are generally old and well known in the art. Circuit breakers are used to protect electrical circuitry from damage due to an over-current condition, such as an overload condition or a relatively high level short circuit or fault condition. In small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light commercial applications, such protection is typically provided by a thermal-magnetic trip device. This trip device includes a bimetal, which heats and bends in response to a persistent over-current condition. The bimetal, in turn, unlatches a spring powered operating mechanism, which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system.
Industrial circuit breakers often use a circuit breaker frame, which houses a trip unit. See, for example, U.S. Pat. Nos. 5,910,760; and 6,144,271. The trip unit may be modular and may be replaced, in order to alter the electrical properties of the circuit breaker.
It is well known to employ trip units which utilize a microprocessor to detect various types of over-current trip conditions and provide various protection functions, such as, for example, a long delay trip, a short delay trip, an instantaneous trip, and/or a ground fault trip. The long delay trip function protects the load served by the protected electrical system from overloads and/or over-currents. The short delay trip function can be used to coordinate tripping of downstream circuit breakers in a hierarchy of circuit breakers. The instantaneous trip function protects the electrical conductors to which the circuit breaker is connected from damaging over-current conditions, such as short circuits. As implied, the ground fault trip function protects the electrical system from faults to ground.
The earliest electronic trip unit circuit designs utilized discrete components such as transistors, resistors and capacitors.
More recently, designs, such as disclosed in U.S. Pat. Nos. 4,428,022; and 5,525,985, have included microprocessors, which provide improved performance and flexibility. These digital systems sample the current waveforms periodically to generate a digital representation of the current. The microprocessor uses the samples to execute algorithms, which implement one or more current protection curves.
Due to improper wiring, faulty terminal or other electrical connections, or a faulty interface of the separable contacts, there could be overheated electrical connections at circuit breaker terminals or overheated separable contacts inside of circuit breakers. This could potentially lead to loss of electrical power, an arc fault or a fire hazard. Circuit breakers including a bimetal thermal trip mechanism or a thermal-magnetic trip mechanism may be able to trip on these types of electrical connection faults since overheating at circuit breaker terminals and/or separable contacts would lead to higher temperatures at the bimetal.
However, no such protection is available when an electronic trip unit is employed since the typical overload or over-current protection is achieved by monitoring current using a current transformer, a Rogowski coil or another current sensor instead of a bimetal thermal trip mechanism.
There is room for improvement in circuit breakers.